The production of aluminium castings using permanent moulds can have three types of metal pouring: with high pressure, by gravity, and with low pressure against gravity, wherein the latter two processes need vents for air outlet.
During the process of filling the mould, the molten metal tends to drag and trap the gases (air) that were present inside the cavity, taking them to the higher regions. This is a natural phenomenon in the casting process by gravity or by low pressure, and it has a tendency of formation of porosity in the part if these gases are not removed by mechanisms for air outlet, namely, vents, as shown in FIGS. 1 and 2.
Thus, the main function of vents is to allow discharge of air and other gases (from liquid metal), preventing at the same time the passage of aluminium, as shown in FIGS. 3 and 4.
As is known to those skilled in the art, after an early period of production the aluminium starts to penetrate in the holes of the vents, clogging the air outlets, therefore causing them to lose their function. The major effect of this scenario is the formation of defects due to porosity or the deformation of the part, compromising its quality and hence reducing the productivity of the castings, according to what is illustrated in FIG. 5.
Known Conventional Types of Vents—State of the Art
Currently, smelters work with three types of vents for permanent moulds; they are:
a) A machined vent with parallel grooves; it was the first type of vent developed for use in permanent moulds. It is manufactured in a simple design due the parallel grooves.
b) A sintered vent with parallel holes; this type of vent is manufactured with air outlets shaped like holes and not grooves, as the machined vents. However, these holes are parallel throughout the length of the vent.
c) A sintered vent with standard conical grooves; this type of vent has holes for exit of air designed like grooves, i.e., similar to the machined vents. In this type of vent the grooves have a small taper inside, instead a single dimension throughout its length (parallel grooves). What characterizes these conical grooves is the direction of the angle used in the constructive disposition: the smallest dimension is on the external face of the groove and it increases toward the interior of the vent.
Results Obtained From the Use of Conventional Vents of the Prior State of the Art
At the time when the aluminum comes into contact with the vent during the mould filling stage, it is in liquid state at a temperature of 740° C. In this condition, the aluminium has fluidity and it can penetrate through the vent holes, causing their clogging with burrs.
The manner in which the smelters evaluate the effectiveness of a vent during the production of castings is related to the ability to maintain the air outlet free, for a greater number of parts produced; therefore, the longer the vents work without become clogged with aluminium, the greater its efficiency.
The resistance to aluminium clogging is analyzed in terms of the constructive arrangement of the holes in each type of vent. FIGS. 12 and 13 exemplify how the aluminium clogs the vent holes after a certain period of production.
Commonly, the smelters are forced to replace the clogged vents almost in a daily basis, because the aluminium difficults the simple cleaning of the holes. Thus, besides the loss of quality of their castings, the smelters are also penalized with higher costs due the consumption of vents with small lifetime. Another important aspect that contributes to the inefficiency of production are the losses summed with frequent stops of the machinery, in order to replace the damaged vents, since this is a work that requires a lot of runtime.